An active matrix display device includes a display element and a transistor driving the display element that are provided in each of pixels thereof. Usable as the display element is a liquid crystal element including a pair of electrodes and a liquid crystal layer provided between the pair of electrodes or an organic electroluminescence element (hereinafter, referred to as an “organic EL element”) including a cathode electrode, an anode electrode and a layer that is provided between the cathode electrode and the anode electrode and contains an organic electroluminescence material. The transistor is formed of an amorphous silicon semiconductor or a polycrystalline silicon semiconductor. Recently, a thin film transistor formed of an oxide semiconductor is also used.
For example, a display device including an organic EL element and a driving transistor that includes a semiconductor layer formed of silicon, a gate insulating layer and a gate electrode is disclosed (e.g., Japanese Laid-Open Patent Publication No. 2007-053286). Also, a display device including an organic EL element and a transistor driving the organic EL element that are integrally formed of an oxide semiconductor is disclosed (e.g., Japanese Laid-Open Patent Publication No. 2014-154382).